World-wide demand for petroleum products has increased dramatically in recent years, depleting much of the known, high value, light crude oil reservoirs. Consequently, production companies have turned their interest towards using low value, heavy oil in order to meet the ever increasing demands of the future. However, because current refining methods using heavy oil are less efficient than those using light crude oils, refineries producing petroleum products from heavier crude oils must refine larger volumes of heavier crude oil in order to get the same volume of final product. Unfortunately though, this does not account for the expected increase in future demand. Further exacerbating the problem, many countries have implemented or plan to implement more strict regulations on the specifications of the petroleum-based transportation fuel. Consequently, the petroleum industry is seeking to find new methods for treating heavy oil prior to refining in an effort to meet the ever-increasing demand for petroleum feedstocks and to improve the quality of available oil used in refinery processes.
In general, heavy oil provides lower amounts of the more valuable light and middle distillates. Additionally, heavy oil generally contains increased amounts of impurities, such as sulfur, nitrogen and metals, all of which require increased amounts of hydrogen and energy for hydroprocessing in order to meet strict regulations on impurity content in the final product.
Heavy oil, which is generally defined as defined as bottom fraction from atmospheric and vacuum distillatory, also contains a high asphaltene content, low middle distillate yield, high sulfur content, high nitrogen content, and high metal content. These properties make it difficult to refine heavy oil by conventional refining processes to produce end petroleum products with specifications that meet strict government regulations.
Low-value, heavy oil can be transformed into high-value, light oil by cracking the heavy fraction using various methods known in the art. Conventionally, cracking and cleaning have been conducted using a catalyst at elevated temperatures in the presence of hydrogen. However, this type of hydroprocessing has a definite limitation in processing heavy and sour oil.
Additionally, distillation and/or hydroprocessing of heavy crude feedstock produce large amounts of asphaltene and heavy hydrocarbons, which must be further cracked and hydrotreated to be utilized. Conventional hydrocracking and hydrotreating processes for asphaltenic and heavy fractions also require high capital investments and substantial processing.
Many petroleum refineries perform conventional hydroprocessing after distilling oil into various fractions, with each fraction being hydroprocessed separately. Therefore, refineries must utilize the complex unit operations for each fraction. Further, significant amounts of hydrogen and expensive catalysts are utilized in conventional hydrocracking and hydrotreating processes. The processes are carried out under severe reaction conditions to increase the yield from the heavy oil towards more valuable middle distillates and to remove impurities such as sulfur, nitrogen, and metals.
Currently, large amounts of hydrogen are used to adjust the properties of fractions produced from conventional refining processes in order to meet required low molecular weight specifications for the end products; to remove impurities such as sulfur, nitrogen, and metal; and to increase the hydrogen-to-carbon ratio of the matrix. Hydrocracking and hydrotreating of asphaltenic and heavy fractions are examples of processes requiring large amounts of hydrogen, both of which result in the catalyst having a reduced life cycle.
Supercritical water has been utilized as a reaction medium for cracking hydrocarbons with the addition of an external source of hydrogen. Water has a critical point at about 705° F. (374° C.) and about 22.1 MPa. Above these conditions, the phase boundary between liquid and gas for water disappears, with the resulting supercritical water exhibiting high solubility toward organic compounds and high miscibility with gases.
Hot pressurized water provides a reaction medium for the heavy components to be cracked into low molecular weight hydrocarbons through facilitating mass diffusion, heat transfer, intra- or inter-molecular hydrogen transfer, stabilizing radical compounds for suppressing coke formation and removing impurities such as sulfur, nitrogen and metal containing molecules. While the exact mechanism of the impurity removal has not been identified, the impurities seem to be concentrated in the coke or heavy fraction of the upgraded products. Through the use of supercritical water, these impurities are oxidized or otherwise modified to avoid deleterious effects. The basic principles of supercritical fluid extraction are outlined in the Kirk Othmer Encyclopedia of Chemical Technology, 3rd Edition, John Wiley & Sons, Supplemental Volume, pp. 872-893 (1984).
However, utilizing supercritical water to upgrade heavy oil can have serious drawbacks. Heavy hydrocarbon molecules dissolute into supercritical water more slowly than their lighter counterparts. Furthermore, asphaltenic molecules, which have a tangled structure, do not untangle easily with supercritical water. Consequently, the portions of the heavy hydrocarbon molecules that do not make contact with the supercritical water decompose by themselves, resulting in large amounts of coke. Therefore, reacting heavy oil with supercritical water using current methods leads to accumulation of coke inside the reactor.
When coke accumulates inside a reactor, the coke acts as an insulator and effectively blocks the heat from radiating throughout the reactor, leading to increased energy costs, since the operator must increase the operating temperature to offset for the build-up. Furthermore, accumulated coke can also increase the pressure drop throughout the process line, causing additional increases in energy costs.
One of the causes of coke formation using supercritical water is attributable to limited availability of hydrogen. Several proposals have been suggested to supply external hydrogen to a feed hydrocarbon treated with supercritical water fluid. For example, hydrogen gas can be added directly to the feed stream. Carbon monoxide can also be added directly to the feed stream to generate hydrogen through a water-gas-shift (WGS) reaction between carbon monoxide and water. Organic substances such as formic acid can also be added to the feed stream to generate hydrogen through a WGS reaction with carbon monoxide, which is produced from decomposition of added organic substances and water. Additionally, a small amount of oxygen can be included in the feed stream to allow for oxidation within the feed matrix for generating carbon monoxide. This carbon monoxide can then be used in a WGS reaction for producing hydrogen. However, feeding any external gas into a liquid stream increases costs and introduces added complexity to the process.
One other possible solution to prevent coke build-up is to increase the residence time of the heavy oil within the reactor to dissolve all hydrocarbons into supercritical water; however, the overall economy of the process would be reduced. Additionally, improvements in reactor design could be helpful; however, this would require large expenditures in design costs and might ultimately not prove beneficial. Therefore, there is a need for a process to facilitate the efficient contacting of heavy oil with supercritical water, which does not result in large amounts of coke or substantial increases in operating costs.
As noted earlier, coking results from the inefficient contact of the heavy hydrocarbon molecules with the supercritical water. Therefore, in order to limit the production of low value coke, it would be advantageous to provide a process that increased the ability of the supercritical water to contact a larger portion of the heavy hydrocarbons.
Furthermore, it would be desirable to have an improved process for upgrading heavy oil with supercritical water fluid that requires neither an external supply of hydrogen nor the presence of an externally supplied catalyst. It would be advantageous to create a process and apparatus that allows for the upgrade of the heavy oil, rather than the individual fractions, to reach the desired qualities such that the refining process and various supporting facilities can be simplified.
Additionally, it would be beneficial to have an improved process that did not require complex equipment or facilities associated with other processes that require hydrogen supply or coke removal systems so that the process may be implemented at the production site.